Compression ignition or auto-ignition of a lean fumigated or lean premixed charge of fuel, once ignited, has the potential to burn while producing fewer pollutants such as oxides of nitrogen (NOx) and particulate matter (PM) than is the case for a non-premixed charge. Controlling ignition of such charges in an internal combustion engine can be difficult, however. Early ignition of a premixed charge during the compression stroke of the piston can result in overpressure, excessive pressure oscillations (typically referred to as engine knock) and subsequent engine damage. As such, conventional internal combustion engine strategies employ premixed charges in cylinders with relatively low compression ratios thus inhibiting both early pre-ignition and what is often referred to as auto-ignition of the end-gas (or portions of the fresh fuel and air mixture that burn late in the combustion cycle). This helps keep pressures and temperatures in the cylinder within a range that provides more control over the combustion rate (hence eliminating or limiting the magnitude of engine knock). These engines typically initiate combustion with a spark plug and rely upon flame propagation to consume the fuel to near completion, theoretically placing a lower limit on how fuel-lean the fresh charge can be without incurring a significant loss in combustion efficiency. However, by limiting the compression ratio and restricting how fuel-lean the premixed charge can be prepared, the performance of the engine can also be limited and result in combustion that can be more polluting than is the case for combustion arising from auto-ignition of a fuel-air mixtures that is prepared with a fuel to air ratio below the theoretical lean propagation limit.
Conversely, premixed charges have been employed in engines with higher diesel-like compression ratios. The fueling strategies employed with such engines seek to avoid excessive knocking that can negatively affect the engines in other ways, however. For example, knock intensity in high compression ratio engines can be limited by limiting the fuel-lean equivalence ratio such that excessive engine knock will not be problematic for the premixed fuel introduced into the combustion chamber. This, however, can affect the engine operating load range. Other strategies affect performance, increase system complexity, and/or increase costs.
One method of utilizing the clean burn of a lean premixed charge while avoiding losses in performance is to combine the method above that employs higher diesel-like compression ratios with a diffusion combustion strategy (see U.S. Patent Application Publication No. US 2002/0078918 A1). As mentioned above, a premixed fuel/air charge (prepared to a fuel-lean equivalence ratio where excessive engine knock will not be problematic) is introduced into a high compression ratio engine. In order to supplement energy limited by the “excessive knock limit” of the premixed charge, additional combustion energy is provided by a quantity of fuel directly injected into the combustion chamber when the piston is at or near top dead center. The resulting burn of the directly injected fuel will, for the most part, be a diffusion burn of this fuel, ignited by compression heating of the premixed charge and potentially the burning premixed charge. Assuming combustion phasing of the premixed charged is controlled, the relatively short and easy to predict delay between the start of the direct injection process and the subsequent ignition of that fuel helps to ensure that the fuel will burn when desired while imparting additional energy to the engine.
A more auto-ignitable pilot fuel can be added and mixed with a premixed fuel/air charge early in the compression stroke to help ensure auto-ignition of the charge (see U.S. Patent Application Publication No. US 2002/0078918 A1 and U.S. Pat. No. 6,230,683). The fueling strategy that employs a premixed fuel/air charge with or without an injection of pilot fuel during the intake or early in the compression stroke, where the early pilot is employed to modify the auto-ignition properties of the premixed fuel/air and where a supplemental quantity of directly injected fuel is also provided to extend the load range of the engine will be referred to herein as a premixed charge combustion ignition engine with direct injection fueling (PCCI-DI) at or near top dead center (see U.S. Patent Application Publication No. US 2002/0078918 A1).
An important issue with PCCI-DI engines is the control of combustion phasing, herein referred to as controlling the start of combustion (SOC) of the premixed charge. That is, while ignition delays are inherent in spark-ignited applications and in applications that utilize a near top dead center (TDC) injection of auto-ignitable fuel to control SOC, these delays are understood in principle, relatively short and, therefore, relatively predictable. However, SOC in an PCCI-DI engine is driven by the properties of the premixed charge as it evolves for each cylinder during an intake and compression stroke, hence it is dictated strongly by variations including, but not limited to, intake temperature and pressure, humidity and fuel quality. The effect of each variable upon the evolution of mixture state results in an onset of combustion that is less easily predicted and controlled. Moreover, system variability can result in fuel/air mixtures with different thermodynamic states for each engine cylinder as the engine cycle evolves. Therefore, the onset of auto-ignition for each cylinder may not occur at the same engine crank angle.
Control of SOC from cylinder to cylinder (that is, cylinder balancing) is important. Therefore, direct SOC monitoring can be valuable. Accurately determining SOC from cycle-to-cycle requires an accurate sensor measurement for determining SOC such as cylinder pressure sensors, acoustic sensors and knock detectors, as well as a computationally efficient method to provide cycle-to-cycle results commensurate with the speed of the engine. As well, the ignition levers employed preferably provide accurate manageable control across the cylinders. Examples of such ignition levers that can help control SOC in PCCI-DI can be things such as direct controls on an ignition source (for example, glow plug temperature) or the timing and quantity of an injection of an auto-ignitable pilot fuel affecting the auto-ignition properties of the premixed charge. Prior, conventional control strategies for a PCCI-DI engine consider such factors as intake charge properties and exhaust gas properties to control the quality of cycle to cycle combustion without monitoring SOC (see U.S. Patent Application Publication No. US 2002/0078918 A1). Greater accuracy is desirable for control of a PCCI-DI. Direct estimation of SOC can provide this.
Herein, the term SOC (start of combustion) is used to refer to an indicator of the combustion phasing of the premixed part of the combustion event which, in general with a PCCI-DI engine, would include the fuel air charge created by the introduction of a first fuel, mixed with an air charge and a pilot fuel within the intake system and/or possibly the combustion chamber such that such premixed (or partially premixed) charge is available near the completion of the compression stroke of the engine or near the beginning of the power stroke of the engine. Persons skilled in the technology involved here will understand that there are many different ways of defining SOC or combustion phasing. Some of these methods include a consideration of the crank angle at which:                peak heat release occurs,        a fraction of the premixed fuel energy has been released (for example, 10%), or        a fraction of the total energy is released.        
Note, for the purposes of this application a premixed charge is a charge that is introduced into the combustion chamber of an engine that is substantially mixed with air or oxygen prior to combustion of the fuel. As such, a premixed charge includes a fumigated charge inducted into the combustion chamber during the intake stroke, a fuel directly injected into an air charge during either compression or intake stroke of the piston, or other method of providing a fuel/air charge to the combustion chamber that is substantially mixed prior to initiation of combustion of that charge. A premixed charge can also include a charge of a mixed first fuel and second fuel and air that is auto-ignitable when the piston is near top dead center. In this case one of the fuels would be more auto-ignitable than the other fuel. A premixed charge will burn in a substantially homogeneous combustion mode.
The present method estimates SOC and applies that estimation to adjust SOC in subsequent cycles of an internal combustion engine. The present method also addresses the issue of computational efficiency.